1. Field of the Invention
The present invention relates to a home agent.
2. Related Art
A handover technique where a mobile terminal switches and uses a plurality of different networks, such as cellular system access networks like a PHS, a GPRS, a CDMA, and an HSDPA and an access network like 802.11 and 802.16, in accordance with a necessity has been known (refer to JP-A 2005-244590 (KOKAI)). The handover technique is generally called a vertical handover technique.
The vertical handover is realized using protocols, such as Mobile IPv4, Mobile IPv6, MOBIKE (IKEv2 Mobility and Multihoming) protocol. Further, it is known that the technique called soft handover reduces the number of packet losses when a terminal switches the network by these protocols.
Soft handover maintains the connection to the previous network for a while even after the terminal is connected to the new network, the terminal maintains a connection with the network before switching for a predetermined time. As a result, the terminal can securely receive all of the packets passing through the network used before switching, while network switching process.
Further, during the handover process, a relay apparatus (home agent) that provides mobility to the terminal also maintains a connection to the previous network after switching the network, for a while. As a result, the relay apparatus can securely receive all of the packets passing through the network before switching, during a network switching process and transmit the packets.
A soft handover method according to the related art at the time of vertical handover using Mobile IPv4 will be described. Hereinafter, the soft handover at the time of a vertical handover is simply referred to as handover. In this case, the case where a mobile terminal switches a used network from a cellular network to a wireless LAN will be described.
Before handover, a tunnel through a cellular network is established between a mobile terminal and a home agent. The home agent receives packets transmitted from a cellular network interface of the mobile terminal and transmits the packet to the counterpart apparatus.
The mobile terminal determines that it switches into a wireless LAN, when signal strength of an electric wave transmitted from a wireless LAN base station becomes sufficiently strong. In addition, a Registration Request message is transmitted from the wireless LAN interface of the mobile terminal to the home agent.
The home agent receives the Registration Request message and detects that the mobile terminal requires switching of the network. Then, the home agent transmits a Registration Reply message, the mobile terminal is allowed to switch network, to the wireless LAN interface of the mobile terminal.
The home agent configures a new tunnel such that the packet transmitted from the wireless LAN interface of the mobile terminal can be received, on receiving the Registration Request message or transmission of the Registration Reply message.
Further, if the mobile terminal receives the Registration Reply message, the mobile terminal configures a new tunnel so that the mobile terminal can transmit the packet from the wireless LAN interface to the home agent. Thereby, the mobile terminal can transmit the packet to the home agent by both of the wireless LAN interface and the cellular network interface.
In general, the communication delay of the cellular network is larger than the communication delay of the wireless LAN. For this reason, at a point of time when the packet is first received through the wireless LAN, the home agent does not receive the packet that is transmitted from the cellular network interface of the mobile terminal.
Even after establishing the new tunnel through the wireless LAN, the home agent holds configuration of the tunnel through the cellular network, and deletes the configuration of the tunnel after a sufficient time passes. Thereby, it is possible to receive all of the packets transmitted from the cellular network interface of the mobile terminal without causing a packet loss at the time of the handover.
However, in the handover method according to the related art, since the home agent cannot detect a point of time of receiving all of the packets transmitted from the cellular network (network before the handover) interface of the mobile terminal, the configuration of the tunnel is not deleted for some time after all of the packets are received, and a memory resource is wastefully used. Since the home agent establishes a tunnel with a plurality of terminals, it is strongly required to release the wasteful memory resource as early as possible and efficiently use the memory resource.